J. Am. Chem. SOC.1991, 113, 4461-4468
446 1
Gas-Phase Reactions of Si+(2P)with Small Hydrocarbon
Molecules: Formation of Silicon-Carbon Bonds
S. Wlodek,t A. Fox, and D.K. Bohme*
Contribution from the Department of Chemistry and Centre for Research in Earth and Space
Science, York University, North York, Ontario, Canada M3J 1P3. Received October 25, 1990.
Revised Manuscript Received February 5, I991
Abstract: Reactions of ground-state Si+(lP) ions have been investigated with the hydrocarbon molecules CH4, C2H6, C2H4,
C2H2,CH2CCH2,CH3CCH, and C4H2proceeding in a helium bath gas at 0.35 Torr and a temperature of 296 f 2 K with
the selected-ion flow tube (SIFT) technique. A range in the nature and degree of reactivity was observed. Methane reacts
slowly to form an adduct ion. Results of quantum chemical calculations performed at the UMP4(SDTQ)/6-31GS*//
UHF/6-3 IG**level are presented which provide insight into the possible structure of this adduct ion. Adduct formation also
is a strong feature in the reactions of Si+ with C2H2and C2H4 in which it competes with condensation resulting in H atom
elimination. The reaction with C2H6 predominantly leads to Si-C bond formation in the product ions together with elimination
of CH3,CHI, or H2. For these three reaction channels possible mechanisms are proposed involving C-C and C-H bond insertion.
The reactions with allene and propyne are rapid and have similar product distributions. Several bimolecular product channels
are observed and all lead to Si-C bond formation. The reaction with diacetylene is unique in that it proceeds rapidly only
by hydride ion transfer. Higher order reactions leading to more complex silicon-carbide ions have also been characterized.
The reactivities of the Si+reactions are compared with those available for the analogous reactions with C+ proceeding under
similar conditions of temperature and pressure. Except for the reactions with allene and propyne, the reactions with Si+are
uniformly slower and less efficient than the corresponding reactions with Cc, and the competition with adduct formation is
not apparent for the reactions with C+ in which charge transfer and H atom elimination are sometimes more predominant.
The observation and failure to observe reactions at 296 K is used to provide insight into the standard enthalpies of formation
of the silicon-containing hydrocarbon ions SiCH2+,SiCH3+,SiC2H+,SiC2Hs+,SiC2H2+,SiC2H?+,SiC2H6+,SiC3H3+.SiC4H5+,
and SiC4H+. A brief discussion is presented of the implications of the chemistry observed with Si+ for the formation of molecules
containing Si-C bonds in partially ionized environments containing silicon and carbon, such as the circumstellar envelopes
of carbon stars.
Introduction
There is a growing interest in fundamental aspects of the
chemical bonding of carbon to silicon. For us this interest has
been stimulated in part by the detection of silicon-carbide molecules in circumstellar envelopes which can be penetrated and
ionized by interstellar UV photons and a need to understand their
chemistry of formation. For example, the silicon-arbide molecules S i c , c-Sic,, and Sic4have recently been identified by
radioastronomers in the circumstellar envelope of the carbon star
IRC+10216 which is famous in astrochemistry for its wealth of
molecular species.'-'
To begin the exploration of Si/C ion
chemistry we have conducted a systematic investigation of ion
chemistry initiated by ground-state atomic silicon ions reacting
with small saturated and unsaturated hydrocarbon molecules.
Here we report the results of measurements taken with a selected-ion flow tube (or SIFT) apparatus of the primary kinetics
and product distributions at room temperature (296 f 2 K) in
helium carrier gas at 0.35 Torr for reactions of Si+(,P) with
methane, ethane, ethylene, allene, acetylene, diacetylene, and
propyne. These particular hydrocarbons were chosen as substrates,
in part, because of the importance of some in the chemistry of
circumstellar envelopes, but also to explore more generally the
ability of silicon to bond to carbon in reactions of Si+with saturated and unsaturated hydrocarbon molecules. An attempt is
also made to track higher order reactions in these gases which
lead to growth of more complex silicon-bearing carbonaceous ions
and molecules.
Several of the hydrocarbon reactions have been investigated
previously with other techniques, but often at nonthermal energies
and without state selection of Si+(2P), and invariably at low
pressures. The reaction with methane which is of interest in C-H
activation has been studied by ion cyclotron resonance (or ICR)
mass spectrometry4 and reported not to proceed at low pressures
at energies near room temperature. A very recent guided ion-beam
mass-spectrometry study has explored in detail the onset of endothermic channels for the reaction of methane with Si+(2P)under
'Present address: Department of Chemistry, Simon Fraser University,
Burnaby, B.C., Canada VSA 1S6.
Chart I
H1
H'\
+
si-
/
H3
/
H2
'
c
H1
single collision conditions from near-thermal to 14 eV relative
kinetic energy.5 Also, an early tandem mass-spectrometer study
(1) Cernicharo, J.; Gottlieb, C. A.; Guelin, M.; Thaddeus, P.; Vrtilek, J.
M. Astrophys. J . (Lutt.) 1989, 341, L25.
(2) Thaddeus, P.; Cummins, S.E.; Linke, R. A. Astrophys. J . (Lett.! 1984,
283, L45.
(3) Ohishi, M.; Kaifu, N.; Kawaguchi, K.; Murakami, A.; Saito, S.;
Yamamoto, S.;Ishikawa, S.;Fujita, Y.; Shiratori, Y.; Irvine, W. M. Astro-
phys. J . (Lett.) 1989, 345, L83.
(4) Stewart, G. W.; Henis, J. M. S.;Gaspar, P. P. J . Chem. Phys. 1972,
57. 1990.
0002-7863191 I1 5 13-4461%02.50/0 0 1991 American Chemical Societv
Wlodek et al.
4462 J . Am. Chem. Soc., Vol. 113, No. 12, 1991
Table I. Rate Constants
cm3 molecule-' C1),Product
Distributions, and Efficiencies for Reactions of Si+(2P)with
Hydrocarbon Molecules at 296 f 2 K
neutral
product
reactant
products
distribution' k,/
k,./kec
CH,
SiCH.+
1.o
0.005 0.00043
0.80
8.0
0.61
C2H6
SiCH? + CH,
0.15
SiCH2++ CH4
0.03
SiC2H4z+ H2
0.02
SiC2H6+
C2H4
SiC2H4
0.60
5.6
0.43
0.40
SiC2H,+ + H
0.70
3.5
0.30
C2H2
SiCiH' + H
SiC2H2+
0.30
0.70
CH2CCH2 SiC3H3++ H
2
0.86
0.20
SiC2H++ CHI,
0.10
SiCH2++ C2H2
0.60
2
0.73
CH3CCH ' SiC,H,+ + H
0.25
SiC2H++ CH,
SiCH2++ C2H2
0.15
CIH2
C4H+-+ SiH- 1 .o
16
1.18
'Primary product ions which contribute 5% or more. The product
distributions have been rounded off to the nearest 5% and are estimated to be accurate to within &30%. *The effective bimolecular rate
constant is given at a total helium pressure of 0.35 Torr and a helium
density of 1.15 X 10l6atoms cm-,. The accuracy of the rate constants
is estimated to be better than *30%. ckcxp/kcis a measure of reaction
efficiency. Collision rate constants, k,, are derived from the combined
variational transition state theory-classical trajectory study of T. Su
and W. J. Chesnavich, J . Chem. Phys. 1982, 76, 5183.
without Si+state selection has been reported for the reaction with
acetylene which was monitored as a function of collision center-of-mass energy in the range 0.3-2.5 eV at a collision chamber
Torr.6 In all three of these studies
pressure of the order of
the ambient pressures were tOo low to cause collisional stabilization
of possible adduct ions. The opportunity for such stabilization
is enhanced under the operating conditions of the SIFT experiments which are reported here. There appear not to have been
any previous reports of the reactions of Si+(zP) with ethylene,
ethane, diacetylene, allene and propyne.
Previous systematic studies of reactions of the same hydrocarbon
molecules with C+ under similar operating conditions allow a
comparison of the reactivities of the group IV atomic ions Si+(2P)
and C+(*P).'** We shall look for differences in the chemical
reactivities of these two atomic ions which may result from the
substantial difference in their standard enthalpy of formation, 136
kcal mo1-l a t 298 K. These should be manifested in the nature
of the bond-formation channels and their relative efficiency, and
also, possibly, in the overall reaction efficiencies.
Experimental Section
All measurements were performed with the selected-ion flow tube
(SIFT)apparatus that has been described in detail elsewhere?JO Atomic
silicon ions were derived from a 2-3% mixture of tetramethylsilane in
deuterium by electron impact at 50-100 eV. The deuterium was added
to scavenge the metastable Si+(4P)ions in the source with the following
reaction4
Si+(4P)+ D2
-
DSi+ + D
(1)
Si+ ions were selected and introduced into helium buffer gas at 0.35 Torr
(or I . I5 X 1016He atoms cm-'). The helium carrier gas and the reagent
gases methane, ethane, ethylene, and acetylene were of high purity (>99.6 mol %). Allene and propyne were of somewhat lower purity (>96
mol 5%). To remove traces of water vapor, the helium carrier gas was
~
(5) Boo, B. H.; Elkind, J. L.; Armentrout, P. 8. J . Am. Chem. Soc. 1990,
112, 2083.
(6) Mayer, T.M.; Lamp, F. W.J . Phys. Chem. 1974. 78, 2645.
(7) Bohme, D. K.;Rakshit, A. B.; Schiff, H. I. Chem. Phys. Leu. 1982,
93, 592.
(8) Herbst, E.;Adams, N. G.; Smith, D. Asirophys. J. 1983, 269, 329.
(9) Mackay, G.1.; Vlachos, G. D.; Bohme, D. K.; Schiff, H. I. Inr. J . Muss
Specrrom. Ion. Phys. 1980, 36, 259.
(IO) Raksit, A. B.; Bohme, D. K. Inr. J . Muss Specrrom. Ion Phys. 1983,
55, 69.
Table 11. Rate Constants (lO-'O cm3 molecule-' s-I), Product
Distributions, and Efficiencies for Secondary Reactions of SiC,H,+
Ions with Hydrocarbon Molecules at 296 2 K
reaction with product distribution'
k,,,b
k,,/k;
*
SiC2H4++ C2H4
2SiC2H,+ + C2H5
2SiC4H8+
-
SiC2H4++ C2H6 products
SiC2H++ C2H2
-
SiC4H3+
4.0
0.36
0.77
0.07
2.0
0.50
-% SiC4H++ H2
SiC2H2++ C2H2 SiC4H4+
2.0
0.50
0.47
SiCH2++ CH2CCH20.95 SiC3H3++ CH3 6.1
0.05
SiC4Hs++ H
SiC2H++ CH2CCH2 SiCsHS+
5.6
0.46
0.61
SiCH2++ CH,CCH 0.85 SiC3H3++ CHI 9.1
SiC4H5++ H
SiC2H++ CH3CCH SiCSHS+
6.5
0.46
'Primary product ions which contribute 5% or more. The product
distributions have been rounded off to the nearest 5% and are estimated to be accurate to within *30%. bThe effective bimolecular rate
constant is given at a total helium pressure of 0.35 Torr and a helium
density of 1.15 X 10l6atoms cm-,. The accuracy of the rate constants
is estimated to be better than *30%. c k c x p / k is
c a measure of reaction
efficiency. Collision rate constants, k,, are derived from the combined
variational transition state theory-classical trajectory study of T. Su
and W. J. Chesnavich, J . Chem. Phys. 1982, 76, 5183.
--
-
passed through zeolite traps (a 5050 mixture of Union Carbide molecule
sieves 4A and 13X) cooled to liquid nitrogen temperature. The reagent
gases were used without further purification. The diacetylene was preand stored at
pared by the alkaline hydrolysis of 1,4-dichlorobut-2-yne'~
dry ice temperature to avoid polymerization. Previous experiments with
H3+as the 'chemical ionization" reagent indicated a purity for the gas
produced in this manner of greater than 99%. All measurements were
made at an ambient temperature of 296 f 2 K.
Experimental Results
Table I summarizes the rate constants and product distributions
observed for the reactions of Si+(2P) with the seven different
hydrocarbon molecules investigated in this study. The measured
rate constants are compared with computed collision rate constants
to provide reaction efficiencies. The reaction efficiency is defined
as the ratio of the experimental to the theoretical rate constant.
Similar data obtained for the secondary reactions which were
observed are summarized in Table 11. In this case experimental
rate constants were determined with a two-exponential computer-fitting technique.
Methane. No bimolecular product channels were observed with
methane. The apparent bimolecular rate constant for the production of the adduct ion SiCH,' was small, only 5 X
cm3
molecule-' s-l. The low magnitude of this reaction rate constant
suggests that the adduct is weakly bound.
Ethane. Si+(2P) was seen to react with ethane much more
efficiently than with methane and a variety of bimolecular products
was observed in addition to a minor channel leading to adduct
formation.
Si+
+ C2H6
0.80
SiCH3+ + CH3
0.15
SiCH2+ + CH,
0.03
SiC2H4++ H2
0.02
__+
SiC2&+
(2a)
(24
All of the bimolecular channels lead to Si-C bond formation.
Formation of SiCH3+ with methyl elimination (reaction 2a)
( 1 I ) Brandsma, L. Prepururiue Acerylene Chemistry; Elsevier Publishing
Co.: New York, 1971; pp 122-124.
Reactions of Si+('P) with Hydrocarbons
106,
I
1
I
I
I
1
si+ +
J. Am. Chem. SOC.,Vol. 113, No. 12, 1991 4463
1
~
2
1
~
106
I j
I
4
10
105
10
3
$
104
z
0
10
103
I
0.6
I
1
I
I
I
I
102
,
E3
E
z
30
1
2
4
C2H2 FLOW
/
6
0
10
12
(molecular roc-1 x 101')
14
Figure 2. The observed variations of ion signals recorded for the addition
of acetylene into the reaction region of the SIFT apparatus in which
Si'(*P) is initially established as the dominant ion in helium buffer gas.
P = 0.351 Torr, B = 7.0 X IO' cm s-l, L = 46 cm, and T = 292 K. The
Si' is derived from tetramethylsilane with added deuterium (3%) at 78
eV.
0.4
and proceeds at about one-half of the collision rate. Both of the
primary ions were seen to react further to form adduct ions by
sequential addition reactions:
0.2
dLL
- - -
SiC2H3+
0.0
I
1
2
3
4
5
6
7
C2H4 FLOW / (molecules sec-lx
8
SiC2H4+
1017)
Figure 1. (a) The observed variations in ion signals recorded for the
addition of ethylene into the reaction region of the SIFT apparatus in
which Si'(*P) is initially established as the dominant ion in helium buffer
gas. P = 0.345Torr, B = 7.0 X IO3 cm s-l, L = 46 cm, and T = 295 K.
The Si' is derived from tetramethylsilane with added deuterium (3%)
a t 72 eV. (b) The fractional abundance of the product ions observed in
part a. The intercepts at zero flow of cyanogen provide a measure of the
initial product distribution, and the ion profiles provide further insight
into the evolution of the products.
predominates over the elimination of methane (reaction 2b) and
hydrogen (reaction 2c). The formation of the adduct ion, SiCZH6+,
is indicative of the importance of the initial Si+-CzH6interaction.
Only the SiC2H4+product ion appeared to react further with
ethane, k = 7.7 X
cm3 molecule-' S-I, but low signal intensities prevented a n identification of the products of this secondary reaction.
Ethylene. Adduct formation and condensation with H-atom
elimination were the only two primary product channels observed
with ethylene as shown in reaction 3. The overall reaction is fast
Si+ + C2H4
0.60
SiC2H4+
(3a)
SiCzH3+
(3b)
0.40
cln4
c2H4
SiC4H7+
Sic.&+
c1n4
CPb
SiC6HII+
(4)
SiCbH12'
(5)
The evolution of the product ions with addition of ethylene is shown
in Figure la. The product fractional-intensity plot in Figure l b
illustrates that SiC2H3+is also produced by the secondary H atom
transfer reaction 6a which predominates over adduct formation.
SiC2H4++ CzH4
08
SiC2H3++ CzH5
(64
0.2
SiCdH8'
(6b)
Acetylene. Acetylene was seen to react with Si+in a manner
analogous to the reaction with ethylene, but in this case H atom
elimination predominates over adduct formation as shown in
reaction 7 and Figure 2. Figure 2 indicates that both product
Si+ + CzHz
0.70
SiC2H+
+H
2SiC2Hz+
(74
(7b)
ions of reaction 7 react further by the addition of a second
acetylene molecule. We have attributed the formation of SiC4H+
to the reaction of SiC2H+ rather than SiC2H2+because of what
is perceived to be a more favorable energetics associated with the
elimination of H2 rather than H, + H, respectively. Analysis of
the data then results in the following reactions and product distributions:
Wlodek et al.
4464 J. Am. Chem. Soc., Vol. 113, No. 12, 1991
SiC2H+
-
+ C2H2 0 9
+
SiC4H3+
(84
-% SiC4H+ + H2
(8b)
SiC2H2+ C2H2
SiC4H4+
0.70 (0.60)
0.10 (0.15)
SiCH2+
+ C2H2(1Oc)
- SiCSHS+
CP4
Sic&,+
(11)
Separate experiments were performed to explore the nature of
the SiC3H3+ion produced from allene and propyne. Formation
of this ion by C-H bond insertion may lead to different isomeric
forms of SiC3H3+which may have different reactivities. As
already indicated, no differences were observed in the reactivity
toward the respective parent molecules. When ammonia was used
as a reactant, two reaction channels were observed, but the
measured rate constants and product distributions again showed
no significant differences for SiC3H3+produced from allene or
propyne. The measured rate constants were equal within experimental error, having values of (7.2f 1.0) X 10-lo and (6.6
f 0.5) X 1O-Io cm3 molecule-' s-l, respectively, where the uncertainties reflect the precision of the measurements. Two product
channels which were observed are shown in reaction 12. NH4+
-
-
SiNH2+ + C3H4
(1 2a)
SiC3H3+.NH3
(1 2b)
was also observed as a product ion, but the contribution of proton
transfer to the loss of SiC3H3+was uncertain due to the presence
of impurity ions which proton transfer to ammonia and because
of the secondary contribution to NH4+ formation from reaction
13 which is known to be rapid.I2 We also report here the inSiNH2+ + N H 3
-
NH4+
+ SiNH
(13)
teresting secondary reaction 14 of SiC3H3+.NH3with ammonia.
The SiN2HS+product ion can be a doubly N-coordinated silicon
species such as (H2N)2SiH+or H3NSiNH2+.
+
'*"H4
SiC2H+ + CH3 (lob)
+
SiC3H3++ NH3
/
H2
+
elimination is the predominant reaction channel, and the ionic
product of this channel, SiC3H3+, is only moderately reactive
toward allene and propyne yielding the adduct ion SiC6H7+. The
other two primary product ions, SiC2H+and SiCH2+,which arise
from CH3 and C2H2elimination, respectively, were observed to
react rapidly with both allene and propyne (seeTable 11). Product
ion distribution plots showed that, while SiC2H+formed mainly
adduct ions with both allene and propyne according to reaction
sequence 1 1, SiCH2+reacted only to form bimolecular products,
SiC3H3++ CH3 and SiC4H5+ H (see Table I1 for rate constants
and product distributions).
CP4
&+.........
SiC3H3+.NH3 NH3
-
+
SiN2H5+ C3H4
(14)
The adduct ions SiC2H+.C3H4and SiC3H3+.C3H4were found
cm3 molecule-I s-l.
to be unreactive toward ammonia, k <
Diacetylene. Hydride abstraction was the only channel observed
for the reaction with diacetylene. The reaction is fast and occurs
(12) Wlodek, S.;Rodriquez C. F.; Lien, M. H.; Hopkinson, A. C.; Bohme,
D. K. Chem. Phys. Lrrr. 1988, 143, 385.
structural parameters
S i 4 = 2.734, C-HI
= 1.098
C-HI = 1.081 1
L(SiCH1) = 57.4
L(SICH,) = 124.1
S i c = 2.295, Si-HI
= 1.534
C-HI = 1.693, C-H2
= 1.077
C-H3 = 1.082
L(SiCHl) = 41.9
L(SiCH2) = 124.3
L(SiCH,) = 92.9
L(H,CSiHl) = 121.8
Si-C = 1.849, Si-H
= 1.471
C-HI = 1.082, C-HZ
= 1.090
L(HSiC) = 120.0
L(H,CSi) = 112.9
L(H2CSi) = 108.3
L(H2CSiH) = 58.2
Si-C = 1.792, Si-H
= 1.455
C-H = 1.077
L(HSiC) = 120.0
L(HCSij = 121.9
H
SiC3H3+ H (loa)
0.20 (0.25)
SiC2H+
state
(symmetry)
isomer
(9)
Figure 2 shows that reactions 8 and 9 proceed with similar rates.
Curve fitting of the intermediate SiC2H+and SiC2H2+ion profiles
provides an effective bimolecular rate constant of 2.0 X 1O-Iocm3
molecule-I S-I for these two reactions.
Allene and Propyne. These two isomeric molecules were observed to react rapidly with Si+ at close to the collision rate to
give multiple bimolecular products with similar product distributions as summarized in reaction 10. Condensation with H atom
Si+ + CH2CCH2(CH3CCH)
Table 111. Computed Structural Parameters for the Three Bound
Adducts of Si+ and CHI and the First Transition State Shown in
Figure 3*
Bond lengths in A and angles in deg.
at. the collision rate. A fast secondaq ieaction of C4H+ was
observed to lead to more carbonaceous ions by addition and
cm3
condensation with elimination of acetylene, k = 2.5 X
molecule-' s-l.
CdH+
+ C4H2 -% CsH3'
2C6H+ + C2H2
(154
(15b)
Both of the products of reaction 15 were observed to add a
molecule of diacetylene to form CloH3+and C12H5+,respectively.
The addition reaction with C6H+was particularly rapid proceeding
nearly at the rate of its production from C4H+.
A previous ICR measurementI3yielded a rate constant of 1.6
X 10-9 cm3 molecule-' s-' for the reaction of C4H+with diacetylene
with the following product channel.
C,H+
+ C4H2
-
CBH2+
+H
(16)
Theoretical Results
Calculations were performed with a GAUSSIAN 8 t program to
gain insight into the nature of the adduct formed in the association
reaction of Si+ with methane.14
The relative stability of three isomers of SiCH4+ and two
transition states for their interconversion were computed at the
UMP4(SDTQ)/6-31G**//UHF/6-3lG**
level. Final energies
were corrected for zero-point energies. The results are summarized
in Figure 3 and Table 111.
Discussion and Conclusions
Methane and Ethane. The low reactivity observed for methane
is in sharp contrast to the high reactivity observed for all of the
other hydrocarbon reactions of Si+(2P) investigated in this study.
Cheng et al. have previously pointed out that no exothermic
bimolecular channels are available to Si+and CH4 at room tem(13) Anicich. V. G.; Blake, G. A.; Kim, J. K.; McEwan, M. J.; Huntress.
W. T. J . Phys. Chem. 1984, 88, 4608.
(14) Binkley, J. S.; Frisch, M.; Raghavachari, K.; DeFrees, D.; Schlegel,
H. B.; Whitside, R.; Fluder, E.; Seeger, R.; Pople, J. A. GAUSSIAN 82, Release
A, Carnegic-Mellon University, Pittsburgh, PA.
J. Am. Chem. SOC.,Vol. 113, No. 12. 1991 4465
Reactions of Si+(lP) with Hydrocarbons
H
Si'
t
C,H,
t4
Si'.. .C,H,
I
14.0
I i.-I
I
H shift
\\
CHS shift
+
H
HSiCH; + CH,
SiCH: + CH,
H
\+
si-C{
'B,
H
Figure 4. Possible mechanism for the reaction of Si+ with ethane initiated
by C-C or C-H bond insertion.
H
of an le, electron of C2H6 onto the half-filled 3p(Si+) orbital and
the destabilization of the C-C and one of the C-H bonds.
Formation of SiCH2+from I requires a four-center transition state
Figure 3. Potential energy profile for the reaction of Si+ with methane
or a rearrangement to 111 with a subsequent passage over a
based on calculations of energies (kcal mol-') for three isomers of SiCH4+
and two transition states for their interconversion. Computations were
three-center transition state, while SiCH3+ results from a simple
level.
performed at the UMP~(SDTQ)~-~I-G**//UHF/~-~ICJ**
Si-C bond scission. This could account for the large value of 5.3
observed for the product ratio of [SiCH3+] to [SiCH2+]. Forperature so that reactions leading to bimolecular products are not
mation of SiCH3+should be preferred over formation of the isomer
expected to 296 K.Is Indeed, a previous study by ion cyclotron
HSiCH2+according to ab initio calculations at the double-{ level'"
resonance (ICR) mass spectrometry has shown that silicon ions
with configuration interaction17bwhich predict that, for singlet
do not react with methane at low pressures and near-thermal
states, HSiCH2+ is 50.3 kcal mol-' less stable than SiCH3+. In
energies4 On the other hand, our calculations indicate that the
fact, these results imply that formation of HSiCH2+ is 43 kcal
"chemical activation" of methane by C-H bond insertion to form
mol-' endothermic while formation of SiCH3+ is 7 kcal mol-'
H-Si+-CH3 is exothermic by 24 kcal mol-' (see Figure 3). This
exothermic. H2 elimination is likely to occur from the insertion
is in accord with the standard enthalpy of formation of 242.6 kcal
intermediate I1 which can pass over the four-center transition state
mol-' reported recently for this ion16 which predicts the C-H bond
IV. Alternately, I1 may rearrange to V and subsequently eliminate
insertion to be exothermic by 35 kcal mol-'. We could therefore
H2. Intermediate V cannot eliminate ethylene since reaction 17
expect a substantial rate of association to form a chemically bound
adduct ion, were it not for the energy barrier of 14 kcal mol-'
which is indicated by our calculations and which will prevent rapid
insertion at room temperature. The experiments indicate that the
adduct ion is formed only very slowly and is therefore likely to
L
be only weakly bound. It corresponds almost certainly to the
IV
V
bridged C , structure (zBI)shown in Figure 3 which has a disis
endothermic
by
12
kcal
mol-'.
Finally,
the
dehydrogenation
sociation energy of only 9.6 kcal mol-'. The computed barriers
of 14.0 and 18.6 kcal mol-' are too high for isomerization to take
Si+ + C2H6 SiH2+ C2H4
(17)
place to the more stable HSiCH3+ (zA') and H2SiCH2+ (zBI)
of I has to be considered. If we assume that the SiC2H4+product
cations, respectively. The weakly bound adduct ion has not been
ion is a C , symmetry species in its ground 2B2state as shown in
observed previously. Our observation of the adduct ion in helium
VI (see next section), its direct formation in a concerted H2
at 0.35 Torr and the failure to observe this ion at the low pressures
elimination from I is unlikely because it is symmetry forbidden.
of the ICR experiments suggest that collisions are required to bring
about its stabilization and so to render it observable.
The Si< bonds formed in the reaction of Si+with ethane may
arise either by C-C bond insertion or by C-H bond insertion
followed by &methyl transfer according to the mechanism posVI
tulated in Figure 4. The nature of the adduct ion, SiC2H6+,is
not known, but its observation as a product ion is consistent with
Ethylene and Acetylene. The adduct ions SiC2H4+and SiC2Hf
the mechanism shown in Figure 4 in which the chemically bound
are important primary products for the reactions of ethylene and
intermediates I and I1 may be collisionally stabilized in competition
acetylene with Si+at the temperature and helium pressure of the
with their decomposition or rearrangement. The C - C and C-H
SIFT experiments reported here. The dominant short-range inbond insertion shown in Figure 4 is easily rationalized in terms
teraction when Si+approaches these two molecules can be expected
of interactions of molecular orbitals. The 3p(Si+) electron can
to involve overlap of the half-filled 3p(Si+) orbital with antibonding
be donated to the 3a2, antibonding ( u * ) molecular orbital of C2H6
A* or bonding A orbitals. Such overlap could result in the forleading to C-C bond breaking with concomitant Si-C bond
mation of symmetric cyclic species of type VI for SiC2H4+and
formation since one of the MO's of the interacting complex has
type VI1 for SiC2H2+. Our preliminary calculations performed
to be bonding and populated with one electron. On the other hand,
at the UMP2/6-31G**//UHF/6-31G**
level suggest that both
an appropriate collision orientation can lead to the delocalization
of these structures are quite stable with De(Si+-C2H4)= 43. i kcal
'A'
-
~
~~~
+
~~
(15) Chen, T. M.H.; Yu,T. Y.;Lampe, F. W.J . Phys. Chem. 1973,77,
2587.
(16) Shin, S. K.;Irikura, K.K..Beauchamp, J. L.; Goddard, W.A., 111
J . Am. Chem. Soc. l!I88,IIO, 24.
(17) (a) Hopkinson, A. C.; Lien, M.H. J . Chem. Soc., Chem. Commun.
1980, 107. (b) A. C. Hopkinson, private communication. (c) Sirois, S.;
Wlodek, S.; Hopkinson, A. C.; Bohme, D. K. Unpublished results from this
laboratory.
Wlodek et al.
4466 J. Am. Chem. Soc., Vol. 113, No. 12, 1991
ii+
Ac
/c
H
H,C
-4'
/
\/
-\
\ /
\
?+
.H
VI1
-.
-
..
,
;i+
>C=CH,
JSi+
C
HjC-
\
'H
VI11
IX
ion is likely to be the protonated linear SiCCHz molecule, although
the formation of the cyclic structure X cannot be excluded without
knowledge of the full potential energy surface of the Si+-C2H4
system. It should be noted that the rearrangement of VI to VI11
could occur driven by entropy unless it is energetically forbidden.
H
I
rise to SiCCCH3+and SiCCH+ respectively, while C-C and Si-C
bond rupture is required for structure XI1 to form SiCHz+.
While the structure of SiC3H3+is unknown, some insight is
provided by the observed reactivity pattern with NH3. The efficient SiNHz+and adduct formation is consistent with a silylene,
R-Si:+, character for this ion since silylene can insert in N-H
bonds. The R' group is likely to be C H 3 m since SiCCCH3+
can be formed directly from structure XIII.
Comparison with C+(2P) Reactions. Due to its much larger
enthalpy of formation (AH01298(C+) = 431.0 kcal mol-',
C+ is much more reactive
cWof,z98(Si+)= 295 kcal
toward most of the neutral reactants employed in this study. The
difference in reactivity is most dramatic with methane for which
no exothermic bimolecular channels exist with Si+. Our calculations show that C-H bond insertion is prevented by an energy
barrier when the semifilled 3p atomic orbital of Si+interacts with
u electrons of CH4. No barrier appears to be present in the
stronger 2p(C+)-u(CH) interaction leading to (HCCH3+!* and
subsequently to (HzCCH2+)*and decay by H and H2 elimination
as shown in reaction 19 which has a rate constant ( k ) of 1.3 X
lo4 cm3 molecule-' s-'.~~ Charge transfer was not observed with
C+ + CH4
sI+
cL='c,
+
+
H
H'
X
The structure of the quenched SiCzH4+which is observed in
our experiments does not necessarily correspond to VI. It could
also have structures VI11 or IX, or even structure XI. XI cannot
Y
R
XI
dissociate at room temperature since the overall reaction (1 8) is
endothermic by about 21 kcal mol-'. However, the occurrence
of the secondary reaction (6) is more consistent with the formation
Si+
+ CzH4
SiHz+
+ C2Hz
(18)
of structure VI11 as a transient intermediate since it can readily
lose a Si-bonded hydrogen atom to an incoming ethylene molecule.
The ground state of the cyclic SiCzHz+cation is 2Al in which
the bonding between Si+ and C2H2 results from ir(CzHz)
3p(Si+) donation. Our preliminary calculations indicate this state
is only 0.5 and 8.5 kcal mol-' more stable than the isomers
SiCCH2+ (2B2) and HSiCzH+ (2A,), respectively." The rearrangement from structure VI1 to SiCCH2+ is therefore energetically allowed. Again, however, the question whether the
SiC2H2+can explore more than one minimum before it dissociates
cannot be answered until information about the full potential
energy surface becomes available. An H atom can be eliminated
directly from structure VII. The closed-shell product ion, SiCzH+,
is a protonated silicon carbide Sic2. The ground state of Sicz
has been shown to be a cyclic T-shaped moleculeZbut the structure
of the protonated form is not known (see Note Added in Proof).
Allene and Propyne. Our experiments have shown that Si+
eliminates an H atom from allene and propyne to form molecular
ions, SiC3H3+,which present the same reactivity toward ammonia.
Therefore it is reasonable to assume that the same isomer of
SiC3H3+is formed in these two reactions. It is likely that Si+
initially interacts with the ir electrons of C3H4leading to cyclic
transient complexes of type XIIand XI11 and that their interconversion (or conversion to a common transient) is responsible
for the nearly identical product distribution which was observed.
Structure XI11 can easily lose an H atom or CH3 radical giving
+
+
s'+
Xlll
XI1
mol-' and De(Si+-C2H2)= 43.7 kcal mol-' (2Al ground state for
VII), r e s p e ~ t i v e l y . 'The
~ ~ ground state of SiC2H4+represented
by structure VI is a 2Bz state which implies that the bonding
between Si+ and CzH4 involves 3p(Si+) ir*(CzH4) electron
backdonation with the main interaction being ir(CzH4) p(Si+)
electron donation. This vibrationally excited state could probably
rearrange to the open structure VI11 or IX which have energies
of only 5.6 and 4.7 kcal mol-', respectively, above the 2B2structure
VI. VI11 in turn can dissociate to H and SiCzH3+. The latter
I
+H
CzHZ+ + H2
C2H3+
(194
( 19b)
Si+ but has been reported for C+ reacting with all of the unsaturated molecules studied here, CzH4, CH2CCH2,CH3C2H,and
C4H2, except C2H2.7J'3 This is consistent with the exothermicity
for these reactions. Adduct ions of the type observed in the
reactions of Si+with Cz.Hz and C2H4 have not been reported for
the corresponding reactions of C+ occurring under similar SIFT
condition^.^,^ However, H atom elimination is an important
competitive channel in the reactions of these two molecules, being
more predominant for the reactions of C+. These differences in
adduct formation and H atom elimination can be accounted for
if the intermediates (C3H2+)* and (C3H4+)* have a shorter
lifetime. We may also note that the product channels in the
reaction of C+ with C2H4 leading to the formation of C3H2+ and
CZH3' do not have their analogues in the reaction of Si+ with
C2H4. In the latter reaction the formation of SiC2H2+,SiCH3+,
or C2H3+ is endothermic.
Allene, propyne, and diacetylene all react rapidly with C+ and
Si+, but there are large differences in the observed product distributions. The reactions of C+ with allene and propyne favor
elimination of H2 and hydride transfer7 while the reactions with
Si+favor H atom and CH3elimination. Only elimination of CzH2
is common to both sets of reactions. The reaction of diacetylene
with C+ favors H atom elimination and charge transfer20 while
with Si+ hydride transfer is the only channel.
Except for the reactions with allene and propyne, the reactions
of Si+are uniformly slower and less efficient than the reactions
with C+, all of which proceed with rate constants of 1.3 X lo4
cm3 molecule-' s-' or higher.7J'*'9
Thermochemistry. The observation and failure to observe reactions at thermal energies at 296 f 2 K can provide useful limits
to the standard enthalpies of formation of the silicon-containing
hydrocarbon ions, most of which are still not well established. The
enthalpies of formation derived from this study are summarized
in Table V. The temperature is assumed to be 298 K. The
required standard enthalpies of formation are taken from Table
1v.
(18) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.;Levin, R.
D.; Mallard, W. G. J . Phys. Chem. ReJ Dora 1988, 17, Supplement NO.1 .
(19) Curtis, R. A.; Farrar, J. M. J . Chem. Phys. 1985, 83, 2224.
(20) Dheandhanoo, S.; Forte, L.; Fox, A,; Bohme, D. K. Can. J. Chem.
1966, 64, 641.
Reactions of Si*(2P) with Hydrocarbons
Table IV. Standard Enthalpies of Formation (kcal mol-I) Used in
the Text'
ion/molecule
-
ref ion/molecule
M01,298
H
108 (2)
b
H+
365.7
b
295 ( i j
b
C+
431.0
b
SiH
90.0 (2.0) b
34.8 (0.3) b
SiH+
272.3 (1.2) c
CH3
-17.8 (0.1) b
274.1
d
SiH2+
CH4
C2H2
54.5 (0.25) b
283.5 (3)
e
SiCHzt
C2H4
12.5 (0.2) b
SiCH3+
233.5 (5)
e
C,Hc
28
b
242.6
d
HSiCHIt
H2SiCH2' 247 (4)
d
C;H;
-20.1 (0.05) b
SiNH,+
21 2.7
f
CHECH,
45.6 (0.2) b
HSiNh+
266.3
CHiCCH44.6 (0.5) b
-1 1.0
b
C4H2
105
h
NH3
104.0 (2)
b
CN
g
HCJN
87
"The stationary electron convention has been adopted for the enthalpies of formation of ions so that these are 1.48 kcal mol-l lower
than values that include the enthalpy of the electron. Uncertainties are
in parentheses when they are available. bReference 18. <Boo, 9. H.;
Armentrout, P. B. J . Am. Chem. SOC.1987, 109, 3549. dReference 16.
e Reference 5. fReference 12.
Reference 23. *Reference 22.
Si
Si+
1
Table V. Standard Enthalpies of Formation (kcal mol-') for
Silicon-Containing Hydrocarbon Ions Deduced in This Study"
AHorm
ion
this work
other values
ref
5
SiCH2+
280 (14)
283.5 (3)
SiCHpt
230 (6)
233.5 (5)
5
SiC2Ht
<288 (3), >255 (2)
<278
21
SiC2H2+
<350 (2)
SiC'H-+
<255 (3), >233 (1)
SiC:H:+
<275 (1)
SiC;H6+
<275 (1)
SiC,H,+
280 (17)
SiC4H+
<343 (3), >348
SiC4H5+
<276 (4)
The stationary electron convention has been adopted for the enthalpies of formation of ions so that these are 1.48 kcal mol-' lower
than values that include the enthalpy of the electron. Uncertainties are
in parentheses when they are available.
SiCH2+,SiCH3+.The observation of the product of these two
ions in the reaction of Si+ with ethane and their failure to be
produced from the reaction of Si+ with methane sets AHOr
(SiCH2+) = 280 f 14 kcal mol-' and AHoI(SiCH3+) = 230 f
6 kcal mol-' which are consistent with the values of 283.5 f 3
and 233.5 f 5 kcal mol-', respectively, determined recently in an
ion beam study of the reaction of Si+with methane by Boo and
co-worker~.~
SiC2H+. This ion has been observed as a product in the reactions of Si+ with C2HZ,CH2CCH2,and CH3CCH. The latter
reaction provides the lowest upper limit of 288 f 3 kcal mol-l to
AHor(SiC2H+). An even lower upper limit of 278 kcal mol-' is
provided by the observation of SiC2H+as a product of the reaction
of Si+ with cyanoacetylene which we have reported previously.21
A lower limit of 255 f 2 kcal mol-' can be deduced from the
failure of the reaction of Si+with C2H4 to produce SiC2H+if this
failure is due to endothermicity.
SiC2H3+. The observation of this ion as a major product in the
reaction of Si+ with ethylene places an upper limit of 255 f 2
kcal mol-' to AHO@iC2H3+). A lower limit of 233 f 1 kcal mol-'
is appropriate if we accept that the non-observation of SiC2H3+
in the reaction of Si+ with ethane is due to endothermicity.
SiC2H2+,SiC2H4+,SiC2H6+. The observation of the formation
of these adduct ions sets upper limits of 350 f 2, 308 f 2, and
275 f 1 kcal mol-' respectively to their standard enthalpies of
formation. The observation of SiC2H4+in the reaction of Si+with
ethane, if it has the same structure as the adduct ion, reduces the
upper limit of AHoI(SiC2H4+)t o 275 f 1 kcal mol-I.
(21) Wlodek, S.; Bohme, D. K. J . Am. Chem. Soc. 1989, 1 1 1 , 61.
J . Am. Chem. SOC.,
Vol. 113, No. 12, 1991 4467
SiC3H3+,SiC4Hs+. The product of these two ions from the
reaction of Si+ with CH3CCH sets the upper limits AHOr
(SiC3H3+)< 293 f 4 kcal mol-' and AHor(SiC4HS+)< 276 f
4 kcal mol-'. A lower limit for AHo&3iC3H3+) = 267 kcal mol-'
may be derived from the production of SiNH2+and propyne from
the reaction of SiC3H3+with NH3. Production of the higher
energy isomer HSiNH+ is probably endothermic since it would
lead to a lower limit for AHor(SiC3H3+)which is 28 kcal mol-'
higher than the upper limit deduced from the reaction of Si+with
propyne. These observation lead to a recommended value for
AHoI(SiC3H3+)280 f 17 kcal mol-I.
SiC4H+. It is interesting to note that SiC4H+ is not produced
from C4H2 while SiC2H+ is readily formed from CzH2. The
former observation implies a lower limit of 348 kcal mol-! to
AHof(SiC4H+)assuming no barrier to reaction. Here we have
adopted a value of 105 kcal mol-' for AHOr(C4H2) which has been
estimated from 'group
With the unknown uncertainty of this limit, this result is barely consistent with the upper
limit of 343 f 3 kcal mol-' which can be deduced from the
observation of SiC4H+as a product of the reaction of SiC2H+with
C2H2when 288 f 3 kcal mol-I is adopted as the upper limit to
the standard enthalpy of formation of SiC2H+derived from the
propyne reaction. The value of AHOF(SiC2H+)= 278 kcal mol-'
derived from the cyanoacetylene reaction leads to an upper limit
of 333 kcal mol-I which is somewhat out of line with the lower
limit of 348 kcal mol-I, presumably because of errors in the
standard enthalpies of formation of both cyanoacetylene, which
is a calculated value and has an unknown ~ n c e r t a i n t y ,and
~~
diacetylene determined from group additivity.22
Circumstellar Chemistry and Molecule Formation. There is a
need to understand the contribution of ion chemistry to the formation of the siliconerbide molecules which have been identified
in partially ionized carbon-rich circumstellar envelopes. The
laboratory identification of new ion-molecule reactions which
trigger the formation of chemical bonds between silicon and carbon
and chemical pathways which lead to further ionic and molecular
growth is therefore valuable, not only in accounting for the known
silicon-carbide molecules but also for the prediction of new circumstellar silicon-bearing molecules and for the assessment of
the early stages of the gas-phase transition from atomic silicon
to silicon carbide particles in such envelopes. Neutral molecules
can be generated by ion chemistry in these environments either
directly as products of ion-molecule reactions or through neutralization reactions such as proton transfer, charge transfer, or
recombination with electrons.24 For example, neutralizatipn of
SiC4H+ may produce the Sic4molecule by proton transfer or
recombination with electrons as shown in reaction 20. A wide
SiC4H++ M, e
Sic4 MH', H
(20)
-
+
variety of silicon-hydrocarbon ions is generated by the ion
chemistry identified in this study, viz. SiCH2+,SiCH3+,SiC2H+,
SiCIHZ+, SiC2H3+, SiC2H4+, SiC3H3+, SiC4H+, SiC4H3+,
SiC4H4+,and SiC5Hs+. The neutralization of these ions provides
possible sources for the neutral molecules SiCH, SiCH2, S i c 2 ,
SiC2H, SiC2H2, SiC2H3, SiC3H2, Sic4, SiC4H2, SiC4H3, and
SiCSH4,respectively. Many of these ions and molecules are novel
and little is known about their structures and energetics. They
need to be investigated theoretically and with other experimental
techniques. Also, in quantitative treatments of this chemistry,
as in the modelling of complex kinetic environments, it must be
recognized that both temperature and pressure may influence
reaction rates and product distributions. For example, as the
pressure drops collisional stabilization will compete less effectively
and adduct formation must proceed by radiative stabilization. We
may also note that adduct ions of the type observed in the case
of methane, viz. Si+.CH4, in which there is a barrier to the formation of a covalent Si-C bond, neutralization by electron/ion
(22) Stein, S.E.; Fahr, A. J . Chem. Phys. 1985, 89, 3714.
(23) Deakyne, C. A,; Meot-Ner, M.; Buckley, T. J.; Metz, R. J . Chem.
Phys. 1987,86, 2334.
(24) Bohme, D. K. In Rote CoeJJcienls in Aslrochemistry; Millar, T. J.,
Williams, D. A,, as.Kluwer
; Academic Press: New York, 1988; pp 117-133.
Wlodek et al.
4468 J . Am. Chem. SOC.,Vol. 113, No. 12, 1991
C2Hz
1
SiC,H'
&- I
-
SIC,
I
)
SiC,H,*
CH,C,H
structure (22) in which the silicon atom is at one end, acetylene
will need to interact with HC+=C=Si: at C+ to form H-C+=
CH-CH=C=Si:
and eliminate HZ,or, alternatively, interact
with H-C=C-Si+ at Si+ to form H-C+=CH-Si-Ce-H
followed by internal electrophilic attack of C+ with the triple bond
to form a five-membered ring and ring opening by breaking the
C-Si bond to form H-C+=CH-CH=C=Si:.
We also note again the possible formation of a doubly N-coordinated silicon ion, (HZN)$SiH+ or H3NSiNH2+,in reaction
14. Neutralization of this ion by proton transfer or recombination
with electrons is likely to lead to the diaminosilene molecule
(NH2)$i:. We note also that the SiNzH5+ion is not directly
accessible in pure ammonia. Sequential reactions of Si+ with
ammonia terminate exclusively and directly in the formation of
the neutral molecule :SiNH as indicated in the following stepsi2
In the presence of allene or propyne the SiN2H5+ion is also
SiC,H,*
Si+ + N H 3
SiNH2+
SiC,H,
recombination is likely to re mstablish the methane molecule together with a silicon atom as in reaction 21. A catalytic role in
the recombination of atomic silicon ions with electrons can
therefore be postulated for methane at low pressures and electron
densities when the adduct ion forms by radiative association. The
complete sequence is given by the following reactions
Si+CH4
Si+
Si+.CH4
Si
e
e
CH,
(2 1b)
Si
In the absence of methane the recombination of atomic silicon
ions with electrons would need to occur radiatively and likely would
have a specific rate much smaller than that for radiative association
in the presence of methane.
We have previously drawn attention to the possible role of silicon
i o n ~ o l e c u l reactions
e
as sources of silicon-carbide molecules in
interstellar and circumstellar environment^.^^.^^ A qualitative
summary of some of the chemistry is given in Figure 5. The
scheme for the production of Sic4which we first proposed in
19MZ5is of particular interest as the Sic, molecule has just
recently been identified in the envelope of the evolved carbon star
IRC+10216.3 Laboratory microwave spectroscopy of a discharge
of silane and acetylene together with quantum chemical calculations have established that this circumstellar molecule is in its
linear *u+ ground electronic state. We note here that the ionic
mechanism which we have proposed may be more favorable at
low pressures at which the collisional association channels 7b and
8a are no longer competitive and all of reactions 7 and 8 may lead
to bimolecular products. The mechanism of formation of the linear
SiC4H+ ion (22), a possible source of the linear Sic, molecule,
from the acetylene reaction of SiC2H+,probably also linear as
shown in (23) (see Note Added in Proof), is intriguing. To produce
H-C+=C=C=C=Si:
H-C+=C=Si:
+ H + SiNH
NH,+
-
(24)
(13)
(25)
established as summarized below. In fact,
molecules initiated by atomic silicon ions. The heavy arrows indicate
neutralization reactions such as recombination with electrons and proton
transfer.
+ - +
+ -
-
+H
NH4+ + SiNH
SiNH2+
+ NH3
Si+ + 2NH3
SiC,H,
Figure 5. Limited reaction scheme for the synthesis of silicon-carbide
Si+ + CH4
--
+ H-C*-C=C-Si+
(22)
+ H-CEC-Si+
(23)
(25)Bohme, D.K.; Wlodek, S.;Fox, A. In Experiments on Cosmic Dust
Analogues; Bussoletti, E., Ed.; Kluwer Academic Press: New York, 1988;
pp 239-244. Bohme, D.K.;Wlodek, S.;Fox, A. In Rafe Coefficients in
Astrochemistry; Millar, T. J., Williams, D. A,, Eds.; Kluwer Academic Press:
New York, 1988;pp 193-200.
(26)Bohme, D. K.; Wlodek, S.;Fox, A. In InfersfellorDust; NASA
Conference Publication 3036;Tielens, A. G. G. M.; Allamandola, L.J., Eds.;
1989;pp 473-478.
+ C3H4
SiC3H3++ N H 3
Si+
SiC3H3+.NH3+ N H 3
Si+ + 2 N H 3
--
SiC3H3+
(1 0 4
SiC3H3+-NH3
(12b)
SiN2H5++ C3H4
(14)
SiNzH5++ H
(26)
the C3H4molecule acts as a catalyst. A similar role for C3H4
is seen in the reaction sequence of eqs 10a and 12a which leads
to the formation of SiNH2+,the precursor of hydrogen silaisonitrile, :SiNH.
Of course the interest in synthetic aspects of ion/molecule
reactions initiated by Si+ and silicon-containing derivative ions
with hydrocarbon molecules is not restricted to extraterrestrial
environments. For example, silane glow discharges with and
without added compounds recently have attracted considerable
attention as a means of producing hydrogenated amorphous silicon
(a-Si:H) films in the semiconductor indu~try.~'In this connection
there is also an interest in the chemistry which may occur at a
silicon surface exposed to hydrocarbon gases. Recent FTMS
experiments have shown the production of the silicon-hydrocarbon
ions SiC2H,+ with x = 1, 2, and 3 when silicon surfaces are
exposed to acetylene and ethylene and vaporized with a laser, but
the nature of the reactions, whether surface or gas phase, is
uncertain.28 Interestingly, no such ions were observed in the
FTMS experiments when methane was deposited, as might be
expected from our results if gas-phase ion/molecule reactions
predominate.
Acknowledgment is made to the Natural Sciences and Engineering Research Council of Canada for the financial support of
this research. We also thank A. C. Hopkinson for helpful discussions.
Note Added in Proof. A recent report on the gas-phase generation and structural characterization of SiCzH and SiCzH+ by
neutralization-reionization mass spectrometry provides evidence
for the existence of silicon acetylide, HC=CSi', and its cation,
H C E C S ~ + . ~The
~ experiments provide support for a b initio
studies at the MP/6-31G**//6-3 1G** level which predict that
(Cmo)is 94 kcal mol-' more stable than +C=CHC=C-Si+
SiH (C-") and that the cyclic C, structure of (SiC2H)+apparently
does not have a minimum.'O
(27)Kruzelecky, R. V.;Racansky, D.;Zukotynski, S.;Gaspari, F.; Ukah,
C. I.; Perz, J. M.J . Non-Crysf.Solids 1989,108, I15 and references therein.
(28)Creasy, W. R.; McElvany, S. W. Sur/. Sci. 1988,201, 59.
(29)Srinivas, R.; Siilzle,D.; Schwarz, H.Chem. Phys. Lett. 1990, 175,
575.
(30)mores, J. R.; Largo-Cabrerizo, A,; Largo-Cabrerizo, J. J . Mol. Srruct.
THEOCHEM 1986,148, 3 3 .